ath11k: debugfs: use ath11k_debugfs_ prefix

As these functions are now defined in debugfs.c change the prefix to use
ath11k_debugfs_ as well.

No functional changes. Compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600264523-12939-2-git-send-email-kvalo@codeaurora.org